1. Field of the invention
This invention relates to the field of electronically tunable optical filters utilizing acousto-optic (AO) diffraction.
2. Description of Prior Art
The acousto-optic tunable filter (AOTF) is an electronically tuned optical filter based on AO diffraction. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the interaction medium. A list of the salient features of the AOTF include wide tuning range, rapid random access tuning rate, high spectral resolution, large angular aperture, inherent modulation and multi-wavelength capability. Due to these attractive properties, the AOTF has found a variety of applications that include electronically tuned spectrometers, spectral imaging, laser wavelength tuning, and more recently, wavelength division multiplexing (WDM) cross-connect in fiber optic communication networks.
The most widely used type of AOTF utilizes a noncollinear AO interaction in a birefringent crystal wherein an incident light of a first polarization is diffracted by an acoustic wave in a birefringent crystal to a second polarization of the light beam. By properly selecting the acoustic frequencies, a number of narrow optical passbands satisfying the phase matching condition are diffracted. Over a wide spectral range, all the diffracted beams at the selected optical frequencies or wavelengths are along a single direction. The AOTF thus has a small input frequency or spectral bandwidth and a small output angular bandwidth. The filtered light beam is separable from the incident light beam spatially or by use of crossed polarizers.
One type of noncollinear AOTF was described in U.S. Pat. No. 4,052,121 entitled xe2x80x9cNoncollinear Tunable Acousto-Optic Filter.xe2x80x9d An important feature of this type of noncollinear AOTF is its large angular aperture. This is due to the choice of an interaction geometry wherein the tangents to the loci of the incident and diffracted light wave vectors are xe2x80x9cparallel,xe2x80x9d a condition known as non-critical phase matching (NPM). Another type of noncollinear AOTF known as the critical phase matching (CPM) type was described in U.S. Pat. No. 3,953,107; entitled xe2x80x9cAcousto-Optic Filter.xe2x80x9d This type of noncollinear AOTF has a small angular aperture and must be restricted to a well-collimated light source. More recently, a new type of CPM AOTF was disclosed in U.S. Pat. No. 5,329,397. In this collinear beam (CB) AOTF, the acoustic group velocity is collinear with the optical beam. The CBAOTF extends the interaction length and realizes high resolution and low drive power.
The AOTFs described above are based on birefringent diffraction that couples incident light to diffracted light with orthogonal polarizations and different refractive indices. This type of AO diffraction can occur only in birefringent crystals. The AOTF is thus inherently sensitive to the polarization of the incident light since it is based on birefringent diffraction. A polarization independent (PI) AOTF using a polarization diversity configuration is disclosed in a co-pending application Serial No. 08/858,093 filed May 17, 1997, now U.S. Pat. 6,016,216, which is incorporated herein by reference. A more common type of AO diffraction is the isotropic diffraction that occurs in isotropic media or in birefringent crystals between incident and diffracted light with the same polarization. Up to now, isotropic AO diffraction has not been used for AOTF applications.
A different class of AO device is the AO Bragg cell (BC) or deflector used for laser scanning. By varying the acoustic frequency the AOBC scans an incident beam into a wide range of resolvable angular positions or spots. As a deflector a basic performance requirement of the AOBC is a large bandwidth, which is defined as the number of resolvable spots per unit time. Thus, in opposition to the AOTF, the AOBC has a large input frequency bandwidth and a large output angular bandwidth.
The realization of a large bandwidth has been one of the major goals in the design of the AO defector. One technique for increasing the bandwidth of the AOBC is the use of acoustic beam steering with a phased array of transducers. The simplest phased array employs a fixed phase difference of 180 degrees between adjacent transducer elements in a planar configuration. By selecting the inter-element spacing to be equal to the characteristic length over a larger frequency range, the bandwidth of the AOBC is thereby increased. This technique is referred to as tangential phase matching (TPM) since the steered acoustic wavevector is tangential to the locus of the diffracted light vector. A more efficient use of the acoustic power has been demonstrated using a stepped phased array where the height of each step in the phased array is equal to xcex9/2. The phased array is blazed so that the beam steering angle from the transducer plane is zero at the reference acoustic wavelength xcex91. Recently a wideband AOBC using acoustic phased array in birefringent crystals was disclosed in U.S. Pat. No. 5,576,880. All of the prior art have limited the discussion on the use of phased array for increasing the bandwidth of AO deflector by operating at the TPM condition, i.e. the steered acoustic wave is tangential to the loci of the diffracted light.
The AOBC can also be used as the dispersive element in an electronically tunable spectrometer for optical spectrum analysis. At a given frequency, the AOBC disperses an input collimated beam of wide band of optical frequencies into a distributions of resolvable angular positions or spots. By varying the acoustic frequency the center wavelength of the entire diffracted spectrum is scanned. This type of AO spectrometer was described in an article entitled, xe2x80x9cColor Control by Ultrasonic Wave Gratings,xe2x80x9d appearing on pages 751-756 in the September 1955 issue of the Journal of the Optical Society of America. Unlike the AOTF, both isotropic and birefringent AO diffraction can be used. An AOBC based spectrometer using birefringent diffraction with a fixed acoustic frequency is described in U.S. Pat. No. 4,639,092.
For use as a wavelength selective cross-connect in WDM networks the prior art AO devices suffer from a basic limitation. The AOTF provides independent and simultaneous selection of multiple wavelengths, but it cannot separate these filtered spectral components. On the other hand, the AOBC separates all of the wavelengths into different directions but cannot independently select the wavelengths of the incident light beam. What is needed but is not yet available in the state-of-the-art is a device with the functional capability of independently and simultaneously selecting and separating multi-wavelength optical beams.
The essence of the subject invention is the discovery that by using acoustic phase array techniques, a new type of AOTF hereafter referred to as the phased array (PA) AOTF can be constructed. The PAAOTF has the narrow input optical passband feature of the AOTF as well as the wide output angular bandwidth characteristic of the AOBC. The PAAOTF provides independent and simultaneous selection and separation of multi-wavelength light beams. Functionally, it acts as a dynamically reconfigurable wavelength division multiplexer or demultiplexer.
One object of the present invention is to provide a preferred configuration of a dynamically reconfigurable wavelength division multiplexer with selectable wavelength and variable amplitude control.
Another object of the present invention is to provide preferred configurations of the PAAOTF for realizing large angular aperture, maximizing spectral resolution, lower drive power, extended optical aperture or minimize tuning speed.